1. Field of the Invention
The present invention generally relates to an apparatus for removing DC offset, and particularly, to an apparatus for removing DC offset in a zero intermediate frequency receiver or a low intermediate frequency receiver.
2. Description of Related Art
A DC offset always occurs in a communication IC receiver configuration when operating at a zero intermediate frequency or a low intermediate frequency. Especially, it happens in a zero intermediate frequency receiver.
An amplifier with a variable gain would saturate when a phenomenon of DC offset occurs, which inhibits the amplifier from achieving its amplifying effect and thus raises a bit-error-rate of the receiver, even resulting in failure of receiving a signal by the receiver.
To improve an effect caused by DC offset in a zero intermediate frequency receiver or a low intermediate frequency receiver configurations, U.S. Pat. No. 4,873,702 discloses segregating DC offset by using an analog\digital converter (ADC) and a digital signal processor (DSP) and then converting the segregated DC offset signal to an analog signal by using a digital\analog converter (DAC), eventually, subtracting an original signal including the DC offset signal from the analog signal, thereby cancelling the DC offset signal.
A circuit for removing DC offset was disclosed in U.S. Pat. No. 5,748,681, wherein a high-pass filter composed of a capacitor with a large capacitance is utilized to remove the DC offset. However, since the two terminals of the capacitor are not connected to ground, it is required to employ a metal-insulator-metal (MIM) capacitor or a polysilicon-insulator-polysilicon (PIP) capacitor with a large capacitance, however these two capacitors occupy a large surface area of an IC substrate and thus cause a problem of an IC design.
Also, U.S. Pat. Nos. 5,793,230 and 5,798,664 disclose a technique of removing the DC offset, which is accomplished by using a low-pass filter composed of a capacitor with a large capacitance. However, since none of the terminal of the capacitor is connected to ground, it is required to employ a MIM capacitor or a PIP capacitor with a large capacitance, however, these two capacitors also occupy a large surface area of an IC substrate and thus cause a problem in the IC design.
In conclusion, although by utilizing the ADC, DSP, and DAC or MIM and PIP capacitors, the DC offset can be removed, the complex circuit design and consumption of large surface area of the substrate by the capacitors make it inconvenient to practically apply in the circuit design.